(1) Field of the Invention
This invention relates to a method for producing an electrical insulating oil composition which is excellent in low-temperature characteristics and which is substantially free from chlorine and chlorine-containing compound.
(2) Description of the Prior Art
West German Laid-Open Patent Publication No. 3127905 discloses that toluene is reacted with benzyl chloride in the presence of an aluminum chloride catalyst in order to prepare an electrical insulating oil composition comprising an alkylated diphenylmethane. The thus prepared electrical insulating oil has excellent low-temperature characteristics, but this method makes use of benzyl chloride as the starting material, and therefore a good deal of chlorine tends to get into the formed electrical insulating oil composition. It is considered that the electrical insulating oil containing much chlorine is poor in stability to oxidation. In addition, when the insulating oil composition lies under a high voltage, chlorine would evolve therefrom and would corrode not only elements of a container for the insulating oil but also the container itself on occasion. Further, at times, the evolved chlorine would react with the insulating oil to inconveniently produce chlorinated aromatic hydrocarbons which are generally believed to have strong toxicity.
Therefore, with regard to the insulating oils, in particular, those composed of aromatic hydrocarbons, the contamination with chlorine must be severely controlled. In short, the technique in the above-mentioned West German publication has the drawback that the insulating oil composition prepared thereby is inevitably contaminated with chlorine.
In the above-mentioned German publication, it is suggested to add thereto an acid acceptor or a stabilizer to oxidation such as a hydroquinone derivative with the intention of preventing troubles due to chlorine. Needless to say, such a measure is not a fundamental solution.